With rapid development in the power supply industry, applications of constant voltage (CV) and/or constant current (CC) switch mode power supplies have increased. Consequently, requirements for CV and/or CC controllers have increased, and power supply manufactures are targeting power supplies with high performance, small volume, and low product costs. As shown in FIG. 1, an example of a conventional secondary controlled CVCC controller for a flyback converter can regulate an output voltage and an output current through sampling resistors, optical coupler 101, and secondary-side feedback controller 102. Sampling resistors can be used to sense output voltage or current, and generate a feedback signal. Optical coupler 101 can be used to transfer the feedback signal to secondary-side feedback controller 102, and second-side feedback controller 102 can be used to control switch QM. In this way, the output voltage and output current can be regulated as substantially constant. However, drawbacks of this approach include high power losses and high product costs, due to feedback components for the secondary-side feedback controller and an optical coupler to transfer the feedback signal.
FIG. 2 shows an example primary controlled CVCC controller for a flyback converter. This flyback converter can include primary winding Np, secondary winding Ns, auxiliary winding NT, main switch QM, current sense resistor Rs, dividing resistors R11 and R22, and controller 201. Auxiliary winding NT can be used to obtain output voltage information, dividing resistors R11 and R22 can be used to divide the output voltage information and generate secondary output voltage feedback signal VFB, and current sense resistor Rs can be used to detect primary current Is and generate secondary output current feedback signal IFB. Controller 201 can be used to receive output voltage feedback signal VFB and output current feedback signal IFB to control a duty cycle of switching signal of main switch QM. As a result, CVCC on the secondary-side can be achieved.
Though the volume and product costs of the example CVCC controller of FIG. 2 can be reduced, other drawbacks remain. For example, during operation of the flyback converter, when the current flowing through secondary winding Ns is reduced to about zero, an induced voltage on secondary winding NT may be significantly decreased, and as a result secondary output voltage feedback signal VFB may not follow variation of the output voltage precisely to yield a large deviation. In this approach, the system may not be precisely controlled. In addition, implementation of the controller may be relatively complicated with a large number of circuit components, because a constant voltage control loop and a constant current control loop controlled by an external compensating circuit may be included.